(A) Field of the Invention
This invention concerns a process for the manufacture of dialkyl phosphorochloridothionate by the chlorination of dialkyl dithiophosphoric acid. The invention further concerns a process for the preparation of dialkyl dithiophosphoric acid by reacting a lower alkyl alcohol with phosphorus pentasulfide.
(B) History of the Prior Art
Dialkyl dithiophosphoric acid (alternatively referred to as O,O-dimethyl-S-hydrogen phosphorodithioate) in the prior art was almost always manufactured in a batch process wherein a lower alkyl alcohol is reacted with solid phosphorus pentasulfide which is frequently slurried into the alcohol in a tank reactor.
Similarly, dialkyl phosphorochloridothionate (alternatively referred to as O,O-dimethyl phosphorochloridothionate) has traditionally been manufactured by chlorination of dialkyl dithiophosphoric acid in a batch process. Such batch processes, have disadvantages of high operation and labor costs and frequently a lower yield. There is now a need for large quantities of dialkyl phosphorochloridothionates for use as intermediates for insecticides such as parathion and as intermediates and for use in other known applications.
Prior art attempts to manufacture dialkyl dithiophosphoric acid and dialkyl phosphorochloridothionate in a continuous operation have had serious disadvantages.
British Pat. No. 745,858 discloses a continuous process for the manufacture of dialkyl dithiophosphates by reacting a slurry of a phosphorus sulfide with an alcohol. The process requires that a portion of the slurry be removed from the reaction vessel followed by separation of dithiophosphate product from the removed slurry. The slurry is then recycled to the reaction vessel. The process requires that slurried materials be removed, transported and maintained in a slurry form which requires expensive equipment and complicates processing.
U.S. Pat. No. 3,897,523 discloses a continuous process for the manufacture of dialkyl phosphorochloridothionates by means of a two step chlorination process. U.S. Pat. No. 3,897,523 discloses that a single stage chlorination uses an excess of chlorine to obtain a reasonable and good yield which lessens overall chlorine efficiency and promotes undesirable side reactions which reduces the product yield. U.S. Pat. No. 3,897,523 also indicates that it is necessary in a one-stage chlorination operation that the single chlorine feed be metered with a high degree of accuracy to properly control the reaction to avoid the yield losses associated with under and over chlorination.
To avoid the disadvantages which were felt to be present in a single stage chlorination operation, U.S. Pat. No. 3,897,523 discloses and claims a continuous process of producing dialkyl phosphorochloridothionates by means of a two-stage chlorination process. The process disclosed in U.S. Pat. No. 3,897,523, while resulting in increased product yield, continued to have substantial disadvantages since the process requires multiple stage reactors which in turn require independent monitoring and control equipment.